Powder metallurgy, i.e., pressing powdered metals in die presses to make parts, is used by a variety of industries as an inexpensive source of parts. Ferrous metal (e.g. iron and steel) parts that are made using present powder metallurgy techniques are however, generally much less than 100% theoretical density at the point they are taken off the powder press ("green" density). For example, typical densities obtained when iron parts are pressed using known methodology can range from 6.4 g/cc to about 7.4 g/cc. In contrast, a fully dense (100% theoretical density) iron part should have a density about 7.8 g/cc.
Many applications for sintered ferrous compacts require higher densities. One reason for this is that a number of properties such as Young's modulus, electromagnetic characteristics and Poission's ratio improves with increased density. A factor contributing to the difficulty in obtaining fully dense ferrous parts using presently available powder metallurgy methods is that particulate ferrous materials "work harden" (although pure iron is generally a ductile material). Additionally, when ferrous powders are pressed, the naturally occurring oxides on the surfaces of the powders to some extent are scraped away and point to point welding occurs between the powders before the part can be pressed to full density and final shape. This density is always less than 100% theoretical. As a result, a much increased energy expenditure is necessary to overcome the results of work hardening and to break the particle to particle bonds, alleviate die wall friction and process the part further. In prior art methods, the powders are lubricated with an organic lubricant prior to pressing as a way to minimize this point to point welding, both powder to powder, and powder to die wall. Typical prior art lubricants include a number of organic compounds such as zinc stearate or waxes. An example of such a lubricant, a polyether lubricant, is taught in U.S. Pat. No. 5,498,276 to Luk. Although the lubrication generally decreases the work expenditure necessary to press the ferrous powders into parts, these parts can also be precluded from being pressed to full density because some degree of internal interconnected porosity must be maintained in the part in order to permit transport of the decomposition products of the organic lubricant inside the part to the outside when it is exposed to elevated temperatures and reducing atmospheres for this purpose. Thus, the resulting green density of such "lubed" parts is generally about 6.3 to about 7.0 g/cm.sup.3. During the "delubing" process, the lubricant is expected to volatize and diffuse out of all portions of the part. This complete volatilization does not however, always occur. The part is then exposed to typical sintering temperatures (about or higher than 0.8 of the melting temperature of the material) in a reducing atmosphere such as dissociated ammonia or hydrogen in order to collapse the porosity. Since the internal lubricants are rarely completely removed, the part is subsequently not completely sintered. Therefore, defects in the parts are common. Additionally, since some of the lubricant remains in the internal porosity of the part overall, properties of the part are degraded.
In addition, complex part shapes such as class 9 and 10 helical gears, other high precision gears, and sprockets with tight dimensional tolerances cannot, in general, be made by powder metallurgy using present techniques because the required high temperature sintering step (to increase the density of the part) distorts the part from its original shape and thus makes it commercially useless. Such complex parts are therefore individually machined using expensive techniques. Thus, there exists a need for methods by which fully dense parts can be made from powdered ferrous materials using less expensive powder metallurgy techniques and for methods by which fully dense parts can be made without using high temperature sintering to avoid distorting the shape of the pressed parts.
Since pure iron is not a generally strong material, in order to increase the strength of parts made from ferrous powders, the methods commonly known as premixing and prealloying are used. Premixing is a method of homogeneously mixing an iron powder with a metal or metalloid powder or an alloy powder, compacting them and subsequently sintering the compact under heat to solid-solubilized these added metals and in some cases added carbon or phosphorus-containing compounds. This method is less than ideal because the added metal powder in the iron powder causes separation or segregation due to the difference between the respective specific gravities and particle shapes of the iron powder and the additive powder(s). This then leads to a problem of part quality by causing wide variability in the strength and the size of the sintered product. U.S. Pat. No. 4,323,395 to Li attempts to address this segregation problem by dipping base metal particles into chemical solutions of "alloying elements" to cover the surface of the particles with these alloying elements. Since this process involves using soluble compounds of the alloying elements for dipping, the resultant coating is not a true metallurgical coating and introduces amounts of contaminants into the compacted part vis-a-vis the chemical compounds (e.g., the anions). Hence, this process is less than ideal for producing high quality parts.
Prealloying involves using an alloyed steel powder in which alloying elements such as nickel, carbon, copper, molybdenum and chromium are solid-solubilized into the iron before compaction. This method is used to avoid the separation problems of premixing. U.S. Pat. No. 5,240,742 to Johnson et al. provides a variation of such a prealloying method. In Johnson et al., iron powders are dipped into a "sol" or solution of chemical precursors of nickel, copper and molybdenum to provide a layer of these compounds on the surface of the powders. The dipped powders are then subjected to a heat and/or reducing treatment to convert the chemical precursors of the metals on the surface of the powders into the metal oxides and to form at least a partially alloyed layer on the surface of the powders. These partially alloyed powders are then compacted and the compacts are subjected to high temperature sintering. This process does however, have its disadvantages. Namely, since the alloyed steel powder obtained by such prealloying processes is relatively hard when compared with pure iron powder, compaction density cannot be increased sufficiently during compaction making it difficult to obtain a green product of high density, hence the subsequent requirement of high temperature sintering. Accordingly, in prealloying processes such as that of Johnson et al., full advantage of the superior physical properties of alloyed steel cannot be taken. Additionally, the chemical precursors of Johnson et al. have the potential to introduce contaminants into the finished pressed part. High temperature sintering also makes the powders and method of Johnson et al. particularly unsuitable for making parts of complex geometry and tight dimensional control.
Thus there further exists a need for a method by which the density, strength and other properties of pressed iron (or ferrous based) parts can be increased which overcomes the disadvantages (as for example, decreased density, introduction of contaminants) of the aforementioned methods.